Conversion coating on metal

ABSTRACT

The quality of a crystalline zinc phosphate conversion coating deposited on a ferrous or zinciferous surface from an aqueous, acidic, non-gassing phosphating solution is improved by the addition thereto of a polymeric additive which is an addition copolymer of an unsaturated carboxylic acid and a selected ethylenic monomer.

United States Patent 3,837,928 Morrison Sept. 24, 1974 [54] CONVERSION COATING 0N METAL FOREIGN PATENTS OR APPLICATIONS [75] lnvemorl Alexander Robley Morris, 45-19366 7 1970 Japan 148/6.15 z

Caulfield, Victoria, Australia [73] Assignee: Dulux Australia Ltd., Victoria, Primary Examiner Ralph Kendal] Austraha Attorney, Agent, or FirmCushman, Darby & 22 Filed: Sept. 12, 1972 Cushman [21] Appl. No.: 288,338

[57] ABSTRACT [30] Foreign Application Priority Data Oct 5 1971 Australia 6526/71 The qualIty of a crystallme zInc phosphate converslon coating deposited on a ferrous or zinciferous surface 52 U.S. cI..... 148/615 2, 117/132 0 260/29.6 M i acid, Pmsphating [51] Int Cl 7/10 lutIon is Improved by the addItIon thereto of a poly- [58] Fieid I6 15 Z meric additive which is an addition copolymer of an 4 unsaturated carboxylic acid and a selected ethylenic monomer.

[56] References Cited UNITED STATES PATENTS 3 Drawmgs 3,728,163 4/1973 Morrison et al l48/6.l5 Z

CONVERSION COATING ON METAL This invention relates to an improved process for the formation of crystalline zinc phosphate coatings on steel and zinc surfaces and to improved zinc phosphating solutions for use therein.

Crystalline zinc phosphate coatings play an important industrial role in up-grading the corrosion resistance and paint adherence properties of ferrous and zinciferous (especially galvanised") surfaces. These coatings, which are applied to the metal by subjecting it to the action of acidic aqueous solutions comprising zinc orthophosphates, are believed to consist essentially of interlocking crystals of insoluble tertiary zinc phosphates, which may contain minor amounts of phosphates of other metals, e.g., iron, calcium and manganese and are known in the art by the trivial name conversion coatings."

Experience has shown that for many applications the best performance in service is given by dense, uniform conversion coatings consisting of fine phosphate crystals, which characteristically have low film weights per unit area of coated surface. In fact, with the introduction of newer paint application techniques, for example electrophoretic deposition, the limitations placed on conversion coating density and effective coating weights appear to be becoming even more stringent.

Broadly speaking, two classes of zinc phosphating solutions are recognised and are reviewed, for example, in an article The Control of Phosphating Solution by D. W. Bloor, in Electroplating and Metal Finishing, July 1964, 235-239. For our present purpose these are conveniently referred to as gassing and nongassing solutions. The gassing phosphating solutions are characterised by the evolution of hydrogen during processing and the formation of relatively heavy coarsely crystalline coatings. The non-gassing solutions contain strong oxidising agents which virtually eliminate gassing and are inherently capable of producing denser conversion coatings of lower coating weights. This invention and the following description are directed to non-gassing zinc phosphating solutions and processes.

The nature of the coating deposited from a particular phosphating solution appears to depend, to a substantial degree, on the metallurgical history of the metal to be coated and its physical pretreatment prior to phosphating. For example, if a conversion coating is applied to a cold-rolled steel surface (pre-cleaned with, for example, trichlorethylene) the coating is usually relatively fine-grained and uniform. If, however, the same surface is precleaned with an alkaline aqueous cleaning bath, which for practical reasons is widely used industrially, the coating deposited thereon from the same phosphating solution is commonly much thicker and comprises relatively coarse crystals. These crystals lead to high coating weights and a fluffy deposit, which often exhibits a pronounced dendritic structure. The films of high coating weight are economically undesirable and show inferior mechanical and corrosioninhibiting properties. Consequently there has arisen a technical and economic need to develop processes which will provide a dense conversion coating of low coating weight and can tolerate the customary cleaning processes of the industry.

It has been proposed that the ability of a phosphating solution to produce dense, fine-grained conversion coatings depends on there being present at the surface of the metal to be treated a substantial number of nucleating sites to initiate rapid crystallisation of the phosphate crystals deposited thereon. Suitable sites are said to be, for example, sharp metal edges, points, heterogeneities at crystal junctions and breaks in the metal crystal texture. It is suggested that the effect of alkali cleaning (and of some acidic media) is to inactivate these sites and hence to lead to the growth of coarser phosphate crystals due to the diminished number of available nuclei. The rate of formation of the coating is also correspondingly reduced.

Since efficient pre-cleaning of metal prior to phosphating is a necessity if a tightly adhering conversion coating is to be formed, various methods have been devised to re-activate the surface after cleaning. For example, it has been proposed to-nucleate the metal surface after cleaning and prior to phosphatng by treating it with a dilute solution of oxalic acid or a complex titanium phosphate. The complex titanium phosphate treatment in particular has been found to be quite effective in producing a finely crystalline phosphate deposit and lowering coating weights. However, this approach can involve introducing one or more additional production stages into the conversion coating process and the imposition of limitations on the composition of the metal pre-cleaning liquids if the effectiveness of the re-nucleating is to be sustained for an economical working life. An alternative proposal is to add materials such as glycerophosphates or certain organic acids, e.g., maleic, citric and tartaric acids, to the phosphating stage. These compounds are reported to have a grainrefining action but for the best results they still appear to require pretreatment of the metal with a complex titanium phosphate. It has also been proposed to induce the deposition of denser conversion coatings by incorporating in the phosphating liquid a cation such as calcium, to modify the crystal growth. The results again are not entirely satisfactory. For example, for some applications the process involves the use of undesirably high processing temperatures.

We have now found that uniform, dense conversion coatings of fine crystal size and unusually low coating weight can be deposited on ferrous and zinciferous metal surfaces using conventional zinc phosphating liquids, provided there is also present in solution in the liquid a polymeric additive as hereinunder definedat a concentration of at least 0.1 per litre. Zinc phosphating baths activated in this way produce the desired coating weights and do not require that the metal be renucleated prior to phosphating, even though it may have been precleaned in an aqueous alkaline degreasing bath. The reason for this effect is not understood and it is surprising that it is only observed when the polymer is present in solution in the actual phosphating liquid, which typically has a pH of about 2 3.5. A prerinse in an aqueous solution of the polymer prior to phosphating has no significant effect on the conversion coating weight or texture.

The particular polymeric additives to be used in this invention are addition copolymers of monomer (a) and monomer (b) in essentially equimolar ratios wherein monomer (a) is at, least one unsaturated polycarboxylic acid or acid anhydride selected from the group consisting of maleic acid, maleic anhydride, fumaric acid and citraconic acid and monomer (b) is at least one unsaturated monomer selected from the group consisting of ethylene, methyl vinyl ether, ethyl vinyl ether and cyclohexyl vinyl ether. The copolymer must be soluble in the phosphating liquid at the concentration required, which is typically 2 gm per litre maximum.

The polymeric additive may be prepared by wellknown convention addition polymerisation techniques. We have found that for ease of processing, copolymers of maleic anhydride are to be desired and hence these are our preferred compositions. These copolymers convert readily, at least in part, to the maleic acid analogues in contact with water. Although the primary characteristic of the polymeric additives we disclose is their molar composition, the solubility requirement imposes certain predictible limitations on their overall nature. For example, it is known that addition polymers of unsaturated monomers are commonly substantially straight-chain polymers in which the molecular weight of the polymer varies with the length of its carboncarbon bonded backbone chain. If the molecular weight of the polymer is too high, it will be insoluble in the phosphating liquid. Polymers of this type are usually prepared by the free-radical initiated polymerisation of the constituent unsaturated monomers and it is known that some degree of branching or even crosslinking of adjacent chains can take place during the polymerisation. This is not objectionable for the performance of the present invention provided the copolymer so-produced is soluble at the required concentration in the phosphating liquid.

it has been observed that at a constant weight concentration in the phosphating liquid, the effectiveness of a polymer as hereinabove defined in reducing coating weights appears to increase with increasing molecular weight of the polymer. If the molecular weight of the selected polymer is too high, the coating weight may be unacceptably low, or in the extreme case no useful coating at all may form. The remedy is then to reduce the concentration of soluble polymer in the coating liquid and/or to select a lower molecular weight analogue of the polymer.

As mentioned above, the minimum effective concentration of polymer in the phosphating liquid is of the order of 0.1 gm per litre, the concentration being adjusted to give a deposited phosphate crystal of the required size and a film of the required coating weight. In general, the most useful range of concentration of polymer in the phosphating liquid is from about 0.25 to 0.5 gm per litre.

The use of the copolymers we have described to control deposited conversion coating weights imposes no unusual requirements on the composition of the actual phosphating liquid. That is, the liquid of the invention is arrived at by simply dissolving a polymer as hereinabove defined in a conventional zinc phosphating liquid at the required concentration and then using the liquid to deposit a conversion coating on ferrous or zinciferous metal in the known manner.

The non-gassing phosphating liquids to which this invention relates consist essentially of solutions in water of zinc orthophosphates and a source of hydrogen ions, commonly phosphoric acid. The liquids as used commercially comprise in addition the appropriate strong oxidising agents, e.gl sodium nitrite and hydrogen peroxide, and are well known and recognisable in the art by terms such as chlorate-nitrite type, which refer to auxiliary features of their mode of functioning. For the purpose of the present invention, however, provided they are essentially acidic zinc phosphating liquids with a pH of about 2 3.5 which deposit tertiary phosphates on the metal to which they are applied, without a significant evolution of hydrogen. we draw no distinction be tween them.

The treatment of ferrous or zinciferous metals to produce a zinc phosphate coating thereon most commonly involves the following sequence of steps; alkali cleaning, water rinse, activating and grain-refining rinse, water rinse, zinc phosphating, water rinse and final passivating rinse. By the use of the above-described polymers, it is possible to reduce these seven stages to live, by eliminating the grain-refining and associated rinse stage. Bearing in mind the consequent reduction in control and capital outlay and decrease in plant size made possible by the present process, its use can represent a substantial economic advantage over existing processes. For some special purposes it may be desirable to retain a grain-refining step, to give coatings of hitherto unattainable density and low coating weight, but in general this is not necessary.

The invention is illustrated by the following examples in which all parts are given by weight:

EXAMPLE 1 Comparison of the conversion coatings deposited on steel and galvanised steel surfaces from a conventional nitrite accelerated zinc phosphating solution and a similar solution according to this invention.

The metals treated were cold rolled steel strip and a similar metal which had been hot dip galvanised. The grade used was known as minimised spangle but not passivated."

The control phosphating liquid was a conventional aqueous zinc phosphating solution and while its composition is not critical within the permitted limitations known to the art, the actual liquid used was prepared according to Formula No. 2 of US. Pat. No. 3,33 3,988. The phosphating liquid according to the invention (hereinunder referred to as the test' liquid) consisted of the above liquid in which was dissolved 0.25 gm per litre of a copolymer of methyl vinyl ether and maleic anhydride in essentially equimolar proportions. The copolymer had a viscosity of 200 centipoise at 25C when tested as a 4 percent by weight solution in water adjusted to pH 9 with sodium hydroxide.

The testing procedure used was to clean the test panels by immersion in an aqueous alkaline cleaning bath of pH about 9, wash thoroughly with water and then treat the cleaned metal surface by spraying it with phosphating liquid at a temperature of 54C for one minute. The panels were then given a further water wash and the coating weight determined by the method of Australian Standard No. SK-9 Part VI i968, Appendix A2 in which the coating is stripped from a weighed panel using chromic acid under standard conditions and the panel re-weighed.

The observed test results were as follows:

The conversion coatings deposited from the test phosphating liquid were visually denser and more finely crystalline than those from the control phosphating liquid and as the above data show their coating weights were substantially lower.

in a further control experiment the methyl vinyl ether copolymer was dissolved at the rate of 0.25 gm per litre in the water of the rinse stage immediately preceding the phosphating stage and the panels then subjected to the action of the control phosphating liquid. The conversion coatings so-formed appeared identical with those produced when no polymer was used in either the water rinse or the phosphating liquid, thus demonstrating not only the effectiveness of the polymer in producing low coating weights but also the need to use it in the phosphating liquid.

EXAMPLE 2 Comparison of the effect of various materials on the conversion coating characteristics of a zinc phosphating liquid.

Separate portions of a conventional zinc phosphating liquid as described in example l were modified by adding to each of them one of the following water-soluble materials at a concentration of 0.25 gm per litre; poly (vinyl alcohol), hydroxyethyl cellulose, poly (vinyl pyrrolidone), sodium poly (acrylate), carboxymethyl cellulose, poly(styrene/maleic acid) and sucrose. The sodium poly (acrylate) carboxymethyl cellulose and the poly (styrene/maleic anhydride) were insoluble in the liquid and these mixtures were not tested further.

Each of the remaining mixtures and a portion of the initial conventional phosphating liquid was then tested for conversion coating properties by the method of example I, when it was seen that none of the additives had an appreciable refining influence on the crystal size or film weight of the coatings so-formed on the metal. It would appear, therefore, that it is not sufficient to just add any soluble high molecular weight organic compound to a zinc phosphating liquid in order to refine the grain-size of the conversion coating formed therefrom in an acceptable manner; the chemical composition of the compound is a critical factor in the process.

EXAMPLE 3 Additive Nature 1 Maleic anhydride/ethylene copolymer (approx. l:l molar ratios) of essentially linear structure and viscosity of 2.0 centipoise as a 2% by wt. solution in water.

2 As for 1 above but viscosity 5.0

centipoise.

3 As for 1 above but viscosity 7.0

centipoise.

poly (methyl vinyl ether/maleic anyhdride) as in example 1.

Citric acid Maleic acid -Continued Additive Nature 7 poly (ethyl vinyl ether/maleic anhydride) 8 I phosphoric acid part ester of poly (vinyl alcohol). 9 poly (vinyl acetate/maleic anhydride) of approximately l:l molar ratio.

The phosphating liquids so-prepared were tested on cold rolled steel plate by the general method described in example 1 save that instead of spraying the phosphating liquid onto the panels, they were immersed in a bath of the liquid held at 667 1C for 3 minutes. The

' No coating.

Thecoating weight in the absence of any additive was approximately 600 mg per sq. ft. of treated metal serface.

Liquids comprising additives Nos. 1-4 and 7, which are compositions according to the invention, effectively reduced the coating weight at concentrations as low as 0.5 gm per litre. The coatings so-produced were dense, uniform and of very fine crystal structure.

The liquids containing additives Nos. 5, 6, 8 and 9, which are not compositions according to the invention, were spot-checked at an intermediate concentration range in the above series. Additives 5 and 6 had no significant effect on the nature of the coating at a concentration of 0.25 gm per litre and while additives 8 and 9 did have a measurable effect at a concentration of 0.5 gm per litre, their usefulness was considerably less than that of the compositions according to the invention and it was even less than that of additives 1-4 when they were used at one tenth of that concentration. A further feature of practical significance in using these compositions can be seen by reference to the test results using additives 1-3. The three additives are basically the same materials but of ascending molecular weights, as indicated by their solution viscosities. The data in the table show that at least for this particular series of resins there is a drop in coating weight with increasing molecular weight of the additive. However, as the results at a concentration of 2.0 gm per litre show, while there may be some advantage (in terms of materials consumed for a given coating weight) if the molecular weight of the resin is too high, an excessive concentration in the phosphating liquid can lead to complete failure to deposit a phosphate coating on the metal. The obvious remedy is to use less additive or to select a lower molecular weight analogue.

What is claimed is:

1. In a process for producing a zinc phosphate conversion coating on a ferrous or zinciferous metal surface by treating the metal surface with an aqueous least one unsaturated monomer selected from the group consisting of ethylene, methyl vinyl ether. ethyl vinyl ether and cyclohexyl vinyl ether,

2. An improved process according to claim 1 wherein the concentration of dissolved polymeric additive in the phosphating liquid is 0.25 0.5 gm per litre.

3. An improved process according to claim 1 in which monomer (a) is selected from maleic anhydride and maleic acid. 

2. An improved process according to claim 1 wherein the concentration of dissolved polymeric additive in the phosphating liquid is 0.25 - 0.5 gm per litre.
 3. An improved process according to claim 1 in which monomer (a) is selected from maleic anhydride and maleic acid. 